Many different types of support piles have been used over the years. Timber was perhaps the first piling material while other materials including steel and concrete were subsequently utilized. Steel piles include HP sections, steel pipe (usually concrete filled) and monotube piles. Concrete piles can be either precast (including both the reinforced and prestressed types) or cast in place. Cast in place concrete piles can further be separated into the non displacement type (auger-cast-pile) or the displacement type. Cast in place concrete displacement piles may be cast directly against the surrounding soil as in the Interpile, Hi-cap, Dewitt, and Atlas piles. Enlarged base piles such as the Franki pile (pressure injected footing) are also cast directly against the surrounding soil. Cast in place concrete displacement piles may also be cast against a metal pipe or metal shell which had previously been bin! drive in to the ground. Metal shells are driven into the ground with the aid of a mandrel such as the step-tapered mandrel (developed by Raymond) or a uniform cross section mandrel (Guiles Mandrel and others). Another type of enlarged base pile the TPT in one form employs a corrugated steel shell secured to the precast tip and internal steel mandrel adapted to be driven by a hammer ram. Some of these pile types will be discussed in more detail below.
Before further discussing the prior art, it will be understood that for a particular project, the particular site geology and structural loads to be borne are considered in conjunction with the cost and load carrying capacity of each of the various pile types and that this analysis usually results in the use of one or at the most two types of the piles. Thus, a pile that can be used throughout a site having different soil conditions provides an advantage over less versatile piles.
In recent years, the cast in place grout piles and, in particular, the augured cast in place pile, has steadily increased in use in the deep foundation market. The auger cast in place pile system uses an auger that is advanced, i.e., screwed directly into the ground. After the auger is advanced to the required depth grout is pumped through the hollow stem of the auger and out through an opening at the tip of the auger. The auger is then pulled out of the ground while grout is released into the hole created by the retreating auger. The auger is not unscrewed from the hole but rather is pulled out in the manner of cork, and the soil that is engaged by and captured in the flights of the auger is removed. Since the soil is removed from the ground and not displaced laterally as in the case of a drive type pile (steel, precast concrete, mandrel driven shells, enlarged base piles etc.) these augured cast in place piles are considered non displacement piles.
Despite the increasing popularity of the auger cast pile, the technique, and other cast-in-place grout piles in general, suffer important disadvantages. These include the following: (1) the inability to fully inspect the completed pile; (2) difficulties in placing reinforcing steel and, in particular, hooked bars or full length reinforcing cages; (3) the integrity of the pile shaft in a variety of soil conditions may be questioned; and (4) the lack of a dynamic procedure by which production piles can be compared to the test pile.
Considering some of these disadvantages in more detail, it is noted that the sides of the hole created during the piling process can push or press in during the removal of the auger and there is no guarantee that the hole will be cylindrical or even that the grout column will be continuous from tip to ground surface. Once the auger is pulled out there can be no inspection of the proper cross sectional area of the pile because the hole is filled with grout, and because of the unknown shape of the pile and the presence of the ever stiffening grout, insertion of a steel reinforcing cage is a problem. Further, with augured cast in place piles, the auger is screwed into the ground to a predetermined depth and because soil conditions can vary widely across a job site, the selected depth may not be optimum at all locations. With driven piles, a test pile is first driven into the ground and resistance to its penetration into the ground is noted. After a load test is conducted on the test pile a relationship between the load carrying capacity of the pile and the resistance to penetration of the pile into the ground is established using any of a variety of generally accepted dynamic formulas. This relationship thus established is used to regulate the installation of subsequent piles at a particular site. The depth to which each pile is driven varies depending on the soil conditions at the individual pile locations. As noted above, with augured cast in place piles there is no generally accepted analytical procedure by which production piles can be compared to a test pile so as to permit a production pile to be tailored to the soil conditions encountered.
Pile systems using mandrel driven shells use both internal and external mandrels. One basic problem with such systems is that of inserting the mandrel into or around the shell and, in particular, the necessity to raise the mandrel above the shell to enable the mandrel to be inserted therein or therearound. For example, assuming that the shell, and thus the mandrel, are fifty feet (50') in height, the upper end of the mandrel must be raised to a height of one hundred feet (100') in order to permit the mandrel to be inserted into a free standing shell. This problem has been overcome to a large extent through the use of so-called "doodle holes" which enable the shell to be inserted in such a previously created oversized hole to substantial depth so as to greatly reduce the height to which the mandrel must be lifted prior to insertion thereof into the shell. The mandrel and shell are then lifted as a unit out of the "doodle hole" for subsequent use. The Raymond "step-tapered" piling system referred to above provides a stepped and tapered "doodle hole."